1. Field of the Invention
Embodiments of the present invention relate to flash memory devices. More particularly, embodiments of the present invention relate to flash memory devices including a multilayer tunnel insulator, and methods of fabricating the same.
2. Description of the Related Art
One important criteria for evaluating properties of flash memory devices is characteristic(s) of a tunnel insulator included therein. Tunnel insulators are insulation layers through which a considerable number of electrons tunnel when programming information to or erasing information from floating gates. Various characteristics of tunnel insulators should be evaluated. For example, the insulating properties, dielectric constant, thickness, flexibility, thermal stability, film composition and density, and, more importantly, compatibility of the tunnel insulator with relatively cheap and/or commonly adopted processes for fabricating typical semiconductor devices should be evaluated. Silicon oxide layers have been widely used as tunnel insulators because silicon oxide layers generally meet the aforementioned requirements associated with tunnel insulators, are widely used in semiconductor processes, and are relatively cheap.
However, as the integration density of flash memory devices has been increasing, the film compositions and structures of flash memory devices have been gradually changing. For example, conductive materials are increasingly being replaced by metallic materials, and structures of films that were previously formed of conductive materials are gradually changing. Various layers other than a silicon oxide layer and a silicon nitride layer are increasingly being used as insulation layers.
Theoretically, as the integration density of flash memory devices increases, the thickness of tunnel insulators should be gradually reduced accordingly. That is, as the integration density of flash memory devices increases, sizes of elements of flash memory devices should be reduced accordingly in order for flash memory devices to operate properly even with low power, and to guarantee stable programming, erasing, and information retention capabilities even at low voltages and low currents.
However, it is not easy to form thinly structured tunnel insulators. From a manufacturing viewpoint, it is relatively difficult to form thin tunnel insulators. From an electrical viewpoint, when tunnel insulators are too thin, electrons stored in floating gates can easily pass through the tunnel insulators and, thus, are likely to leak from the tunnel insulators, thereby compromising image retention capabilities. Thus, a tunnel insulator should be formed to have an appropriate electrical thickness.
It is difficult, however, for conventional tunnel insulators, formed of silicon oxide to meet the requirements of facilitating the tunneling of electrons and enabling information program/erase-operation states even at low voltages, while having stable information retention capabilities.
The properties of tunnel insulators can be improved by using methods that involve forming a thin insulation layer having a high dielectric metal oxide such as hafnium oxide, aluminum oxide, titanium oxide, tantalum oxide, or lanthanide compounds. However, these metal oxides are neither materials that are widely used in the fabrication of semiconductor devices nor materials that are common and plentiful. In addition, it is relatively unstable and costly to form tunnel insulators using these metal oxides. Moreover, tunnel insulators that include these metal oxides generally provide poor interfacial properties with silicon substrates or other conductive materials. Furthermore, tunnel insulators that include these metal oxides are vulnerable to heat, and thus, are generally difficult to apply to the fabrication of semiconductor devices.